Parallel

The Power of "In Progress"

Hold no hostages: Know when to design for "partially updated" as the normal state

Herb Sutter is a bestselling author and consultant on software development topics, and a software architect at Microsoft. He can be contacted at www.gotw.ca.

Don't let a long-running operation take hostages. When some work that takes a long time to complete holds exclusive access to one or more popular shared resources, such as a thread or a mutex that controls access to some popular shared data, it can block other work that needs the same resource. Forcing that other work to wait its turn hurts both throughput and responsiveness.

To solve this problem, a key general strategy is to "design out" such long-running operations bysplitting them up in to shorter operations that can be run a piece at a time. Last month in Break Up and Interleave Work to Keep Threads Responsive, we considered the special case when the hostage is a thread, and we want to prevent one long operation from commandeering the thread (and its associated data) for a long time all at once. Two techniques that accomplish the strategy of splitting up the work are continuations and reentrancy; both let us perform the long work a chunk at a time, and in between those pieces our thread can interleave other waiting work to stay responsive.

Unfortunately, there's a downside: We also saw that both techniques require some extra care because the interleaved work can have side effects on the state the long operation is using, and we needed to make sure any interleaved work wouldn't cause trouble for the next chunk of the longer operation already in progress. That can be hard to remember, and sometimes downright complicated and messy.
Is there another, more general way?

Let It "Partly" Be: Embrace Incremental Change

Let's look at the question from another angle, suggested by my colleague Terry Crowley: Instead of viewing partially-updated state with in-progress work as an 'unfortunate' special case to remember and recover from, what if we simply embrace it as the normal case?

The idea is to treat the state of the shared data as stable, long-lived and valid even while there is some work pending. That is, the "work pending" that results from splitting long operations into smaller pieces isn't tacked on later via a black-box continuation object or encoded in a stack frame on a reentrant call; rather, it's promoted to a full-fledged, first-class, designed-in-up-front part of the data structure itself.

Looking at the problem this way has several benefits. Perhaps the most important one is correctness: We're making it clear that each "chunk" of processing is starting fresh and not relying on system state being unchanged since a previous call. In [1], we had to remember not to make that implicit assumption when we resumed a continuation or returned from a yield; this way, were are explicit about the "data plus work pending" state as the normal and expected state of the system.

This approach also enables several performance and concurrency benefits, including that we have a range of options for when and how to do the pending work. We'll look at those in more detail once we've seen a few examples, but for now note that they include that we can choose to do the pending work:

with finer granularity, so that we hold exclusion (e.g., locks) for shorter times as we do smaller chunks of the work;

asynchronously, so that it can be more easily performed by another helper thread; and/or

lazily, instead of all up front.

Note the "and/or" -- one or more of these may apply in any given situation. Some of these techniques, notably enabling lazy evaluation, are well-known optimizations for ordinary performance, but they also directly help with concurrency and responsiveness.

The rest of this article will consider two mostly orthogonal issues: First, we'll consider different ways to represent the pending work in the data structure, with real-world examples. Then, we'll separately consider what major options we have for actually executing the work, how to use them singly or in combination, and what their tradeoffs are and where each one applies.

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